A surrounding element of the type initially described, which is used in manufacturing an end seal of a shielded synthetic-insulated single-conductor cable with operating voltages of 10 kV or 20 kV, respectively, is described in EP 0 875 087 B1. To produce a cable head of a 20 kV cable on a cable end, at first wires of a cable shielding are removed from a cable insulation provided between a cable conductor and the shielding and are fixed to form a coil of wires, i.e. wires having a common ring-like edge. A conductive layer, e.g. a graphite coating, on the cable insulation is removed as far as an offset edge projecting over the coil of wire. Then the cable insulation provided on the end of the cable conductor is removed and a field-controlling, tubular surrounding element is placed onto the cable end. This surrounding element is formed by a field control element made of a polymer and a microvaristor based on filler containing doped zinc oxide and by an outer layer surrounding the field control element coaxially made of heat-shrinkable insulating material. With its one end the field control element contacts the end of the conductive layer guided as far as the offset edge. The other end of the field control element is guided out over the offset edge to the cable insulation and, depending on the control task, can be connected to the open end of the cable conductor, or can also end on the cable insulation without connection to the cable conductor.
The structure and properties of a field control element used in the above-mentioned surrounding element are described in detail in a publication by R. Strümpler et al. “Smart Varistor Composites”, Proceedings of the 8th CIMTEC Ceramic Congress and Forum on Materials Symposium (1994). The field control element is designed as composite material and is filled with a ceramic material, consisting of small—substantially spherical—ZnO particles. The ZnO particles are doped with different metal oxides, such as e.g. Sb2O3, Bi2O3, Cr2O3 and Co3O4 and are sintered at temperatures of between 900° C. and 1300° C. Like a varistor the sintered particles have nonlinear electrical properties that depend on the electrical field strength. In the case of low field strengths the particles behave like an insulator, and with increasing field strength the particles become more conductive. Owing to these nonlinear electrical properties the polymer composite material has good field-control properties.
A further surrounding body for surrounding a connection point of two high voltage cables having different diameters is described in U.S. Pat. No. 6,171,669. This surrounding body has on its inner side a layer of field control material. This layer is guided on both cables in each case to a transition area which is arranged between the associated cable insulation and a layer made of semiconductive material.